A heat development image forming process has already been proposed, and is described in U.S. Pat. Nos. 3,152,904, 3,457,075, and B. Shely "Thermally Processed Silver Systems" (Imaging Processes and Materials, Neblette eighth edition, edited by Sturge, V. Walworth and A. Shepp, page 2, 1996).
A heat developable light-sensitive material used in the image forming process generally has a light-sensitive layer, which contains a catalytically active amount of a photo catalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., organic silver salt) and a color toning agent dispersed in a binder matrix. The color toning agent has a function of controlling color tone of silver. The process comprises steps of imagewise exposing to light the heat developable light-sensitive material, and then heating the light-sensitive material at an elevated temperature (not lower than 80.degree. C.) to cause an oxidation-reduction reaction between the silver halide or the reducible silver salt (functioning as an oxidizing agent) and the reducing agent. Thus a black silver image is formed. The oxidation-reduction reaction is accelerated by a catalytic function of a silver halide latent image formed at the exposing step. Accordingly, the black silver image is formed within the exposed area. The heat developable light-sensitive material usually contains a synthetic polymer as a binder because gelatin, which is used as a binder of a conventional silver halide photographic material has a poor resistance to heat.
A heat development does not require processing solutions of a wet development. The heat development can easily and rapidly conducted, compared with the wet development. However, a wet development is still conducted in the technical field of photography. The heat development has unsolved problems, while the wet development does not have the problems.
A photographic material usually contains a dye, such as a filter dye, an antihalation dye or antiirradiation dye. The dye functions at the exposing step. If the dye remains in the photographic material after the exposing step, a formed image would be colored with the dye. Therefore, the dye should be removed from the photographic material at a developing step. At the wet development, the dye can easily be removed from the photographic material by using processing solutions. On the other hand, it is very difficult (substantially impossible) to remove the dye at the heat development.
A simple, easy and rapid development has been desired in the field of recent photography, especially in the field of recent clinical or printing photography. The improvement of the conventional wet development, however, has nearly reached its limits. A recent clinical or printing photographic material is imagewise exposed to near infrared laser (particularly electromagnetic wave having a wavelength of not less than 750 nm) or near infrared light emitting diode. If a near infrared light is used, a near infrared absorbing dye is incorporated into the photographic material to prevent halation or irradiation. The near infrared absorbing dye has small absorption in the visible region. Even if the near infrared absorbing dye remains in the heat developing process, influence of the dye on the image is relatively small.
For reasons mentioned above, much attention has been paid again to a heat development image forming process in the field of clinical or printing photography. The near infrared absorbing dyes used in the heat developable light-sensitive materials include cyanine dyes (described in Japanese Patent Provisional Publication No. 8(1996)-201959 and European Patent No. 714046), indolenine cyanine dyes (described in Japanese Patent Provisional Publication No. 4(1992)-182640 and U.S. Pat. No. 5,545,515) and dihydroperimidine squarylium dyes (described in U.S. Pat. No. 5,380,635). On the other hand, cyanine dyes (described in U.S. Pat. No. 5,330,884, European Patent No. 342576A, and Japanese Patent Provisional Publication Nos. 5(1993)-216171, 5(1993)-216167 and 6(1994)-43583) are usually used as the near infrared absorbing dyes in conventional silver halide photographic materials.